Reverse Idler Gear

To engage each gear the gear selector fork moves one of the pink dog-clutches into engagement with one of the blue output gears, to lock it to the output shaft and provide drive through to the prop-shaft.

The additional purple idler gear between the red laygear and the blue reverse output gear reverses the direction of rotation of the output shaft when in reverse.

In practice, and in order to get a 1:1 ratio for 4th gear, there is no laygear for fourth - the dog clutch engages directly with the green input shaft. 3rd gear is the other side of the 4th gear dog clutch, with 2nd and 1st gears with their own dog clutch further along from that.

An oddity with this drawing is that despite each successive forward gear being larger on the laygear and smaller on the output shaft as one would expect, the laygear reverse gear is only very slightly smaller than the output gear size. This would give about a ratio close to 1:1 i.e. 4th gear. In practice reverse is usually a higher ratio than first - 4.76:1 for reverse on the MGB and 3.64:1 for 1st gear:

Output/mainshaft with equal 1st and reverse gears, and the '4/3' synchro and dog clutch assembly moving left for 4th and right for 3rd, ditto the '2/1' assembly:

The laygear has 1st and reverse about the same size on the laygear:

The reverse idler gear having equal input and output gears:

Juxtaposed. The reverse idler gear shown in its engaged position, with the input meshed with the laygear and it's output with the mainshaft. When disengaged the reverse idler moves to the right, into the main bearing cover cutout on the right: